(7/4. 'Beitrage
zur Kenntniss' etc. page 117 et seq. The whole subject of the sterility of
plants from various causes has been discussed in my 'Variation of Animals and
Plants under Domestication' chapter 18 2nd edition volume 2 pages 146-56.) I
mention this case as it appears to be one of compensation. So again is the well-
known fact, that plants which increase largely by stolons or other such means
are often utterly barren, with a large proportion of their pollen-grains in a
worthless condition.
Hildebrand has shown that with hermaphrodite plants which are strongly
proterandrous, the stamens in the flowers which open first sometimes abort; and
this seems to follow from their being useless, as no pistils are then ready to
be fertilised. Conversely the pistils in the flowers which open last sometimes
abort; as when they are ready for fertilisation all the pollen has been shed. He
further shows by means of a series of gradations amongst the Compositae, that a
tendency from the causes just specified to produce either male or female
florets, sometimes spreads to all the florets on the same head, and sometimes
even to the whole plant (7/5. 'Ueber die Geschlechtsverhaltnisse bei den
Compositen' 1869 page 89.); and in this latter case the species becomes
dioecious. In those rare instances mentioned in the Introduction, in which some
of the individuals of both monoecious and hermaphrodite plants are
proterandrous, others being proterogynous, their conversion into a dioecious
condition would probably be much facilitated, as they already consist of two
bodies of individuals, differing to a certain extent in their reproductive
functions.
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